How can i add cess tax 12% after igst 28% in my purchase n sale bill

Good day, @abhisekbegani-gm.

You can create a custom rate in QuickBooks Online (QBO). That way, you can add a cess tax in your purchases. 

Here's how:
 

1. Go to Taxes.
2. Select Add Tax.
3. Click Tax rate.
4. Enter the name of your new tax rate.
5. Select your tax agency.
6. Choose where this tax rate applies for Sales or Purchases transactions.
7. Enter the tax rate.
8. Click Save.

To add, here's an article you can read to learn more about how to enter a new tax rate: Create Custom Tax Rate.

From here, you'll have to select the recently created tax rate in each line item of your sales or purchase transactions. That way, it'll reflect the correct data on your report. You can read through this article to learn more: The GSTR3B (GSTR 3B) report in QuickBooks.

Lastly, I've got you this helpful article for ideas about how you can pay your taxes: How to record tax payments and match them with bank transactions?

If there's anything else that I can help you with, let me know in the comments, @abhisekbegani-gm. I'll be here to lend a hand. Have a great weekend.